Membrane Systems Containing An Oxygen Transport Membrane And Catalyst, And Methods Of Using The Same

ABSTRACT

An apparatus for separating oxygen from an oxygen-containing gas and facilitating a chemical reaction with the separated oxygen includes a mixed conducting membrane, a porous body, and a material for catalyzing the reaction. The mixed conducting membrane has respective oxidation and reduction surfaces and is made of a non-porous, gas-impermeable, solid material capable of simultaneously conducting oxygen ions and electrons. At least the membrane and the catalyzing material are non-reactive with each other or are physically separated from each other during oxygen separation and the chemical reaction. The apparatus can be embodied by tubes, and a plurality of such tubes can form part of a reaction vessel in which various chemical reactions can occur benefiting from the apparatus design.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates generally to the design and use of catalyticmembrane reactors. More specifically, the disclosure relates to reactorscontaining one or more oxygen transport membranes and a catalyst, andmethods of using the same for carrying out more efficient chemicalreactions.

2. Brief Description of Related Technology

Catalytic membrane reactors using solid state membranes for oxidationand/or decomposition of various chemical compositions have been studiedand used. One potentially valuable use of such reactors is in theproduction of synthesis gas. Synthesis gas is a mixture of carbonmonoxide (CO) and molecular hydrogen (H₂), and is used as a feedstock inthe production of bulk chemicals such as, for example, methanol, aceticacid, ammonia, oxo-products, hydrogen, hydroquinone, ethanol, ethylene,paraffins, aromatics, olefins, ethylene glycol, Fischer-Tropschproducts, substitute natural gas, and other liquid fuels, such asgasoline.

Synthesis gas typically is produced from natural gas (i.e., gascontaining methane (CH₄)) or other light hydrocarbons by steam reformingor partial oxidation. In steam reforming, natural gas is mixed withsteam and heated to high temperatures. Thereafter the heated mixture ispassed over a catalyst, such as nickel (Ni) on aluminum oxide (Al₂O₃),to form synthesis gas:CH₄+H₂O→3H₂+COSynthesis gas is obtained in a partial oxidation reaction when naturalgas is reacted with molecular oxygen (O₂) in an exothermic reaction(i.e., the reaction evolves energy:CH₄+½O₂→2H₂+CO

Both the steam-reforming reaction and the partial oxidation reaction areexpensive to maintain. Conventional steam-reforming techniques presentsignificant obstacles. First, the chemical reaction to produce thesynthesis gas from natural gas and steam (H₂O) is endothermic (i.e., thereaction requires energy). Roughly one third of the natural gas consumedin the steam-reforming process is required to produce the heat necessaryto drive the endothermic reaction, rather than to produce the synthesisgas. Second, the ratio of H₂:CO in the synthesis gas produced by steamreforming is relatively high (e.g., about 3:1 to about 5:1). For mostefficient use in the synthesis of methanol, for example, the ratio ofH₂:CO in synthesis gas should be about 2:1. Adjusting this ratio,however, adds to the cost and complexity of the process. In the partialoxidation reaction, significant energy and capital are required toprovide the molecular oxygen necessary to drive the reaction. The oxygentypically is obtained through capital intensive air-separation units.

Catalytic membrane reactors are valuable in the production of synthesisgas. In a catalytic membrane reactor that facilitatesoxidation/reduction reactions, a catalytic membrane separates anoxygen-containing gas from a reactant gas which is to be oxidized.Oxygen or other oxygen-containing species (e.g., NO_(x) or SO_(x)) arereduced at a reduction surface of the membrane to oxygen ions (O²) thatare then transported across the membrane to its other surface, incontact with the reactant gas. The reactant gas, for example methane, isoxidized (e.g., from CH₄ to CO) by the oxygen ions, and electrons (e′)are evolved at the oxidation surface of the membrane. Use of thesecatalytic membrane reactors is believed to be beneficial for a number ofreasons. First, the reaction to produce synthesis gas mediated by thecatalytic membrane reactor (CH₄+½O₂→2H₂+CO) is exothermic, as notedabove. The evolved heat can be beneficially recovered in a co-generationfacility, for example. Second, the synthesis gas produced using thecatalytic membrane reactor can produce synthesis gas having a H₂:COratio of about 2:1. Thus, the additional and expensive processing stepsnecessary in conventional steam-reforming techiques are obviated, andall of the natural consumed gas can be used to produce synthesis gas.

Membrane materials useful in separating oxygen from oxygen-containinggases generally are mixed conductors, which are capable of both oxygenion conduction and electronic conduction. The driving force of theoverall oxygen transport rate through the membrane is the differentoxygen partial pressure applied across the membrane. Suitable membranesare dense and gas-impermeable. Thus, direct passage of oxygen moleculesand any other molecular species through the membrane is blocked. Oxygenions, however, can migrate selectively through the membrane. Themembrane thus separates oxygen from other gases.

More specifically, at elevated temperatures, generally in excess of 400°C. suitable membrane materials contain mobile oxygen ion vacancies thatprovide conduction sites for the selective transport of oxygen ionsthrough the membrane. The transport through the membrane material isdriven by the ratio of partial pressure of oxygen (P_(oxygen)) acrossthe membrane, where oxygen ions flow from a side with high P_(oxygen) toa side with low P_(oxygen). Dissociation and ionization of oxygen (O₂ toO²) occurs at the membrane cathode (or reduction) surface whereelectrons are picked up from near surface electronic states. The flux ofoxygen ions is charge-compensated by a simultaneous flux of electroniccharge carriers in the opposite direction. When the oxygen ions travelthrough the membrane and arrive at the opposite anode (or oxidation)surface of the membrane, the individual ions release their electrons andrecombine to form oxygen molecules, which are released in the reactantgas steam and the electrons return to the other side through themembrane.

The permeation or diffusion rate (also referred to herein as “flux”)through a non-porous ceramic membrane is controlled by (a) thesolid-state ionic transport rate within the membrane, and (b) the ionsurface exchange rate on either side of the membrane. The flux of thegas to be separated usually can be increased by reducing the thicknessof the membrane, until its thickness reaches a critical value. At abovethis critical value, the oxygen flux is controlled by both the ionsurface exchange kinetics and solid state ionic transport rate. Belowthe critical thickness, the oxygen flux is mainly dominated by its ionsurface exchange rate. Therefore, thinner membranes are desirable due tothe higher solid state ionic transport rate than are thicker membranes.However, a lower ion surface exchange rate (i.e., a higher surfaceresistance to transport rate) of thinner membranes, can become dominantin the overall component transport rate. Surface resistance arises fromvarious mechanisms involved in converting the molecules to be separatedinto ions or vice-versa at both surfaces of the membrane.

Oxygen ion conductivity in a material can result from the presence ofoxygen ion defects. Defects are deviations from the ideal composition ofa specific material or deviations of atoms from their ideal positions.One mechanism of oxygen ion conduction in a material is “jumping” ofoxygen ions from site-to-site where oxygen vacancies exist. Oxygenvacancies in a material facilitate this “jumping” and, thus, facilitateoxygen ion conduction. Oxygen ion defects can be inherent in thestructure of a given material of a stoichiometry and crystal latticestructure, or created in a membrane material through reactions betweenthe membrane material and the gas to which it is exposed under theoperating conditions of the catalytic membrane reactor. In a givensystem with a given membrane material, both inherent and induced defectscan occur.

Materials with inherent oxygen anion vacancies are generally preferredfor use as the membrane. Loss of oxygen from a membrane material byreaction to create vacancies typically has a detrimental effect on thestructural integrity of the material. As oxygen is lost, the size of thecrystal lattice increases on a microscopic level. These microscopicchanges can lead to macroscopic size changes. Because membrane materialsare brittle, size increases lead to cracking making the membranemechanically unstable and unusable. Furthermore, the cracking and sizechanges can undesirably render a once gas-impermeable material gaspermeable.

Catalysts useful in the production of synthesis gas are known, and havebeen coated onto surfaces of membranes in the past such as, for example,in Mazanec et al. U.S. Pat. Nos. 5,714,091 and 5,723,035, and inSchwartz et al. U.S. Pat. No. 6,214,757. Generally, such catalystsinclude, but are not limited to, cobalt and nickel on aluminum oxide ormagnesium oxide. These catalysts, however, have not necessarily beenused in combination with catalytic membrane reactors for the productionof synthesis gas.

The beneficial use of catalytic membrane reactors is not limited to theconversion of natural gas to synthesis gas. These reactors also can beused where oxides of nitrogen (NO_(x)) and sulfur (SO_(x)) and hydrogensulfide (H₂S) are decomposed, such as disclosed in the '757 patent.

There are a number of significant challenges in constructing andmaintaining catalytic membrane reactors not adequately addressed in theprior art. For example, membrane materials must be capable of conductingoxygen ions while also being chemically- and mechanically-stable at thehigh operating temperatures and other harsh conditions experiencedduring reactor operation. Further, the membrane material must remainnon-reactive or inert with respect to the various catalyst materialwithin the reactor used to catalyze the chemical reaction. Stillfurther, the membrane material must remain non-reactive or inert withrespect to the various non-oxygen-containing reactants within thereactor consumed in the chemical reaction. Additionally, provisionsshould be made in the reactor for electronic conduction to maintainmembrane charge neutrality.

SUMMARY OF THE DISCLOSURE

An apparatus for separating oxygen from an oxygen-containing gas andfacilitating a chemical reaction with the separated oxygen includes amixed conducting membrane, a porous body, and a material for catalyzingthe reaction. The mixed conducting membrane has respective oxidation andreduction surfaces and is made of a non-porous, gas-impermeable, solidmaterial capable of simultaneously conducting oxygen ions and electrons.At least the membrane and the catalyzing material are non-reactive witheach other or are physically separated from each other during oxygenseparation and the chemical reaction.

In an alternative embodiment, the material for catalyzing the reactionis disposed between the reduction surface and the porous body.Furthermore, each of the membrane, the catalyzing material, and theporous body is constructed of different substances, and the catalyzingmembrane is non-reactive with the membrane at conditions experiencedduring the oxygen separation and the chemical reaction.

In another alternative embodiment, the material for catalyzing thereaction is disposed between the reduction surface and the porous body,but not in physical contact with the reduction surface. This alternativeembodiment optionally includes one or more spacers disposed between thereduction surface and the catalyzing material. Furthermore, themembrane, the catalyzing material, and the porous body are constructedof different substances.

In yet another alternative embodiment, the apparatus includes the mixedconducting membrane and a porous body that itself includes a materialfor catalyzing the reaction, wherein the porous body is disposedadjacent to the reduction surface. The membrane material and catalyzingmaterial are different from each other and are non-reactive with respectto each other at conditions experienced during the oxygen separation andthe chemical reaction. In this embodiment, the membrane material andcatalyzing material are different from each other and are not inphysical contact with each other.

In yet another alternative embodiment, the apparatus includes a mixedconducting membrane, a porous body, and a material for catalyzing thematerial. In this embodiment, the porous body is disposed between thereduction surface and the catalyzing material. Additionally, themembrane, the catalyzing material, and the porous body are constructedof different substances.

In still another embodiment of the disclosure, the apparatus includes(a) a multilayered, mixed conducting membrane that includes a non-porouslayer and a porous layer, wherein the membrane is made of a non-porous,gas-impermeable, solid material or mixture of solid materials capable ofsimultaneously conducting oxygen ions and electrons; and, (b) a materialfor catalyzing the reaction, wherein the catalyzing material is disposedwithin pores of the porous layer. In this embodiment, the catalyzingmaterial is non-reactive with the membrane material at conditionsexperienced during the oxygen separation and the chemical reaction.

Additional features of the disclosure may become apparent to thoseskilled in the art from a review of the following detailed description,taken in conjunction with the drawing figures and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a complete understanding of the disclosure, reference should be madeto the following detailed description and accompanying drawings wherein:

FIG. 1 illustrates a partial, fragmentary, cut-away view of an apparatusaccording to the disclosure;

FIG. 2 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 3 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 4 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 5 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 6 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 7 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 7A illustrate a close-up view of a portion of the apparatus shownin FIG. 7;

FIG. 8 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 9 illustrates a fragmentary, cross-section of a portion of anapparatus according to the disclosure;

FIG. 10 illustrates a perspective view of a multiple-membrane catalyticreactor according to the disclosure.

While the disclosed apparatus and method are susceptible of embodimentin various forms, there is illustrated in the drawing figures and willhereafter be described specific embodiments of the disclosure, with theunderstanding that the disclosure is intended to be illustrative, and isnot intended to limit the disclosure to the specific embodimentsdescribed and illustrated herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An apparatus for separating oxygen from an oxygen-containing gas andfacilitating a chemical reaction with the separated oxygen includes amixed conducting membrane, a porous body, and a material for catalyzingthe reaction. The mixed conducting membrane has respective oxidation andreduction surfaces and is made of a non-porous, gas-impermeable, solidmaterial capable of simultaneously conducting oxygen ions and electrons.At least the membrane and the catalyzing material are non-reactive witheach other or are physically separated from each other during oxygenseparation and the chemical reaction.

Referring now to the drawing figures, wherein like reference numbersrepresent identical elements or features in the various figures, FIG. 1illustrates a partial, fragmentary, cut-away view of an apparatus 20 forremoving oxygen from an oxygen-containing gas. The apparatus 20 includesa mixed conducting membrane 22, a porous body 24, and a material 26 forcatalyzing the reaction. As shown in FIG. 1, the porous body 24 isdisposed between the membrane 22 and the catalyst 26. In operation, anoxygen-containing gas would occupy and pass within a first zone 27defined by the membrane 22, and specifically a reducing (or reduction)surface 28 thereof, while a reactant gas would be present in a secondzone 29 outside the membrane 22, adjacent an oxidizing (or oxidation)surface 30 thereof. Reference herein to surface 28 as the reducing orreduction surface refers to its role in reducing the oxygen in theoxygen-containing gas, whereas reference to surface 30 as the oxidizingor oxidation surface refers to its role in oxidizing the reactant gas.Thus, generally, the oxidizing side of the membrane is the side at whicha process gas is oxidized by oxygen ions separated by the membrane. Thereducing side of the membrane is the side at which a process gas isreduced by removal of oxygen from gas. Geometries and designs in whichthe oxidizing and reducing surfaces of the membrane (30 and 28,respectively) are reversed, such that the reactant gas occupies thefirst zone 27 and the oxygen-containing gas occupies the second zone 29are also contemplated. As set forth in more detail herein, thedisclosure is not limited to these arrangements as other arrangementsare contemplated and are within the scope of this disclosure, such asthose illustrated in FIGS. 2 through 10.

For example, in an alternative embodiment, the material for catalyzingthe reaction is disposed between the reduction surface and the porousbody. FIG. 2 illustrates a fragmentary, cross-section of a portion of anapparatus 32 according to this alternative embodiment. As shown in FIG.2, the catalyzing material 34 is disposed between the reduction surface36 of the membrane 38 and the porous body 40. In this embodiment, eachof the membrane 38, the catalyzing material 34, and the porous body 40are constructed of different substances, and the catalyzing material 34is non-reactive with the membrane 38 at conditions experienced duringthe oxygen separation and the chemical reaction.

In another alternative embodiment, the material for catalyzing thereaction is disposed between the reduction surface and the porous body,but not in physical contact with the reduction surface. FIG. 3illustrates a fragmentary, cross-section of a portion of an apparatus 42according to this alternative embodiment. As shown in FIG. 3, thecatalyzing material 44 is disposed between the reduction surface 46 ofthe membrane 48 and the porous body 50. This alternative embodimentoptionally includes one or more spacers (not shown) disposed between thereduction surface 46 and the catalyzing material 44. Furthermore, themembrane 48, the catalyzing material 44, and the porous body 50 areconstructed of different substances. In this embodiment, the catalyzingmaterial 44 and the membrane 48 can be reactive with respect to eachother at conditions experienced during the oxygen separation andchemical reaction.

In yet another alternative embodiment, the apparatus includes the mixedconducting membrane and a porous body that itself includes a materialfor catalyzing the reaction, wherein the porous body is disposedadjacent to the reduction surface. FIGS. 4 and 5 illustrate afragmentary, cross-section of a portion of an apparatus 52 according tothis alternative embodiment. As shown in FIG. 4, the apparatus 52includes the membrane 54, and a porous body 56 that in itselfconstructed either partially or completely of a catalyzing material. Thematerials comprising the membrane 54 and the catalyzing material aredifferent from each other and are non-reactive with respect to eachother at conditions experienced during the oxygen separation and thechemical reaction. Alternatively, and as shown in FIG. 5, the materialcomprising the membrane 54 and the catalyzing material are differentfrom each other and are not in physical contact with each other.Optionally, one or more spacers (not shown) may be disposed between thereduction surface of the membrane 54 and the porous body 56, wherein thespacers are non-reactive with the membrane 54 material and the porousbody 56.

In yet another alternative embodiment, the apparatus includes a mixedconducting membrane, a porous body, and a material for catalyzing thereaction. In this embodiment, the porous body is disposed between thereduction surface and the catalyzing material. FIGS. 6 and 9 illustratea fragmentary, cross-section of a portion of an apparatus 58 accordingto this alternative embodiment. As shown in FIGS. 6 and 9, the porousbody 60 is disposed between the reduction surface 62 of the membrane 64and the catalyzing material 66. Additionally, the membrane 64, thecatalyzing material 66, and the porous body 60 are constructed ofdifferent substances. In this embodiment, the porous body 60 optionallycan be contiguous with either or both of the catalyzing material 66 andthe membrane 64. Alternatively, the catalyzing material 66 and theporous body 60 are not in physical contact with each other. Optionally,one or more spacers (not shown) may be disposed between the porous body60 and the catalyzing material 66, wherein the spacers are non-reactivewith the catalyzing material 66 and the porous body 60.

In still another embodiment of the disclosure, the apparatus includes(a) a multilayered, mixed conducting membrane that includes a non-porouslayer and a porous layer, wherein the membrane is made of a non-porous,gas-impermeable, solid material or mixture of solid materials capable ofsimultaneously conducting oxygen ions and electrons; and, (b) a materialfor catalyzing the reaction, wherein the catalyzing material is disposedwithin pores of the porous layer. FIGS. 7 and 8 illustrate afragmentary, cross-section of a portion of an apparatus 68 according tothis alternative embodiment. FIGS. 7A and 8A illustrate close-up viewsof portions of the apparatus shown in FIGS. 7 and 8A, respectively. Asshown in FIGS. 7, 7A, 8, and 8A, the apparatus 68 includes amultilayered, membrane 70 that includes a non-porous layer 72 and aporous layer 74. The catalyzing material 76 is disposed within pores ofthe porous layer 74. In these embodiments, the catalyzing material 76 isnon-reactive with the membrane 70 material at conditions experiencedduring the oxygen separation and the chemical reaction.

Shown in FIGS. 1 through 10 are various embodiments of portions of anapparatus according to the disclosure. The embodiments shown are tubularin nature, but need not be and can be of any suitable geometric shape orshapes. Examples of suitable geometric designs include a shell-and-tubedesign (such as the one described herein), a fixed-tube-sheet design, aplate-type design (e.g., plate-and-frame or planar arrangements likethose disclosed in EP 732,138 B1), a bayonet-tube design, a spiral tubedesign, and other designs commonly found in the heat-transfer equipmentarts.

In practice, a reactor will include an arrangement of a plurality of oneor more of the aforementioned embodiments. For example, referring toFIG. 10, there is depicted a reactor 80 utilizing apparatus (in the formof closed tubes 78) of the types described above and depicted in FIGS. 1through 9. The tubes 78, which comprise closed-end membrane tubes, likethose depicted in FIGS. 1 through 9, are enclosed in a reactor module80, and are linked together by a manifold 82. An inlet feed tube 84delivers an oxygen-containing gas 86 to the closed tubes 78, andoxygen-depleted gas 88 exits the tubes 78 via the manifold 82 throughexit tube 90. A reactant gas 92 is delivered to the reduction zone 94via a reactor shell inlet port 96. Reactant gas 98 exits the reductionzone 94 via an outlet port 100.

The term “mixed conducting membrane” as used herein defines a solidmaterial or mixture of solid materials which simultaneously conductsboth oxygen ions and electronic species (e.g., electrons). Additionally,the membrane promotes the coupled reduction of an oxygen-containing gasand oxidation of a reactant gas. The membrane can include any suitablesolid material which perform these simultaneous functions. Suchmaterials generally are described, for example, in Mazanec et al. U.S.Pat. Nos. 5,306,411 and 5,702,959. Carolan et al. U.S. Pat. No.5,712,220, Prasad et al. U.S. Pat. No. 5,733,435, and in Mazanec,Electrochem. Soc. Proceedings 95:16-28 (1997), all of which areincorporated herein by reference.

Alternatively, the mixed conducting membrane can be a mixture of one ormore ion-conducting, solid materials and one or more solid materialswhich conduct electronic species (e.g., electrons) wherein the mixtureof solid materials forms a composite, mixed conducting membrane. Oneexample of a composite, mixed conducting membrane uses zirconia as theoxygen ion-conducting, solid material and palladium as the conductor ofelectronic species. Another example of a composite, mixed conductingmembrane uses zirconia as the oxygen ion-conducting, solid material anda mixture of indium and praseodymium oxides as the conductor ofelectronic species. Since the reactive environment on the reduction sideof the membrane typically creates very low partial oxygen pressures,chromium-containing perovskites are especially suitable materials sincesuch materials tend to be stable in the low partial oxygen pressureenvironment. In contrast, the chromium-containing perovskites typicallydo not decompose at very low partial oxygen pressures.

Preferably, the membrane is a perovskite having the general formula:ABO₃.In the formula, element A is selected from the group consisting of GroupII metals, calcium, strontium, barium, yttrium, lanthenum, lanthanideseries metals, actinide series metals, and mixtures thereof. Element Bis selected from the group consisting of iron, manganese, chromium,vanadium, titanium, copper, nickel, cobalt, and mixtures thereof.

Membranes according to the disclosure are shaped to have two surfaces: areduction surface and an oxidation surface. These membranes can befabricated in a variety of shapes appropriate for a particular reactordesign, including disks, tubes, closed-end tubes, or as reactor coresfor cross-flow reactors. The membrane is fabricated sufficiently thickto render it substantially gas-impermeable and mechanically stable towithstand the stresses associated with reactor operation, yet not sothick as to substantially limit the oxygen permeation rate through themembrane. The membrane preferably has a thickness (T) defined by thedistance between the reduction and oxidation surfaces of about 0.001millimeters (mm) to about 10 mm, more preferably about 0.05 mm to about0.5 mm.

The membrane preferably is capable of transporting oxygen ions andelectrons at the prevailing oxygen partial pressure in a temperaturerange of about 350° C. to about 1200° C. when a chemical potentialdifferences is maintained across the membrane surface. The chemicalpotential difference can be caused by maintaining a positive ratio ofoxygen partial pressures across the membrane. This positive ratiopreferably is achieved by reacting transported oxygen with anoxygen-consuming for reactant) gas. The oxygen ion conductivitytypically is in a range of about 0.01 to about 100 S/CM, where S(“Siemens”) is reciprocal ohms (1/Ω). In addition to the increasedoxygen flux, the membrane preferably exhibits stability over a broadtemperature range of about 0° C. to 1400° C. (and more preferably about25° C. to 1050° C.), and an oxygen partial pressure range of about1×10⁻⁶ to about 10 atmospheres (absolute) without undergoing phasetransitions.

In a catalytic reactor useful for oxidation/reduction reactions, themembrane forms a barrier between an oxygen containing gas and a reactantgas with the reduction surface of the membrane in contact with theoxygen-containing gas in the first zone 27, for example, and theoxidizing surface of the membrane in contact with the reactant gas inthe second zone 29, for example. The oxygen-containing gas is reduced atthe reduction surface of the membrane generating oxygen anions (O²⁻) atthat surface, which are conducted through the membrane in the oxidizingsurface of the membrane. Oxygen anions (O²) at the oxidizing surfaceoxidize the reactant gas, generating electrons (e⁻) at that surface,which are conducted back through the membrane to maintain electricalneutrally in the membrane and the facilitate additional reduction andoxygen anion conduction.

In a catalytic reactor for oxygen separation, the membrane forms abarrier between an oxygen-containing gas, in contact with the reductionsurface of the membrane, and an oxygen-depleted gas or partial vacuum incontact with the oxidation surface of the membrane. Oxygen is reduced atthe reduction surface to form oxygen anions, which are transportedacross the membrane, oxidized at the oxidizing surface of the membraneand released into the oxygen-depleted gas or partial vacuum. Theoxygen-depleted gas does not substantially react with oxygen anions.

Examples of catalytic membrane reactions facilitated by use of themembrane and reactors of this disclosure include partial oxidation ofmethane, natural gas, light hydrocarbons, other gaseous hydrocarbons,and mixtures of methane or other hydrocarbons with or without carbondioxide (CO₂) to synthesis gas, full or partial reductive decompositionof oxides of nitrogen (NO_(x)), oxides of sulfur (SO_(x)), CO₂, andhydrogen sulfide (H₂S), and the separation of molecular oxygen (O₂) frommixtures of other gases, particularly its separation from air. Catalyticmembranes used in accordance with the disclosure can facilitate thereduction of NO_(x) to molecular nitrogen (N₂), SO_(x) to sulfur (S),CO₂ to carbon monoxide (CO), and H₂S to S and H₂O. These types ofmembranes also can be used to facilitate dehydrogenation andoxydehydrogenation reactions of the type disclosed in Mazanec et al.U.S. Pat. No. 5,306,411, the disclosure of which is incorporated hereinby reference.

According to the disclosure, selection of the catalyzing materialdepends upon the reaction desired in the second zone 29 of the reactor.Typically, however the catalyzing material comprises one or more activemetals selected from the group consisting of iron, ruthenium, cobalt,rhodium, nickel, palladium, platinum, copper, silver, gold, and mixturesthereof.

The catalyzing material should have a surface area (A_(e)), and a ratioof active metal to surface area of at least about 0.001 grams per squaremeter (g/m²), preferably at least 0.05 g/m², and more preferably atleast 0.25 g/m². Alternatively, where the membrane reduction surface hasa surface area defined as A_(P), the catalyzing material should have asurface area (A_(C)) defined by the formula:A _(C) =y(A _(R)),where y is 0.01 to 1000, preferably 1 to 100. Considered in another way,the catalyst surface area (A_(C)) should be at least about 0.1 squaremeters/gram (m²/g), preferably at least about 0.5 m²/g, more preferablyat least about 1.0 m²/g, and even more preferably at least about 10m²/g.

Suitable catalyzing material for use in the production of synthesis gasincludes one or more active metals selected from the group consisting ofiron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper,silver, gold, and mixtures thereof. Preferably, the catalyzing materialis about 1 wt % to about 20 wt. % nickel, based on the total weight ofcatalyzing material. More preferably, the catalyzing material is about 7wt % to about 15 wt % nickel, based on the weight of catalyzingmaterial.

Examples of suitable catalyzing materials for use in the full or partialreductive decomposition of NO_(x) to nitrogen and oxygen, SO_(x) tosulfur and oxygen, and hydrogen sulfide to hydrogen and sulfur, and forthe dehydrogenation and oxyhydrogenation reactions are disclosed in, forexample, Mazanec et al. U.S. Pat. No. 5,306,411, the disclosure of whichis incorporated herein by reference.

The catalyzing material can be fabricated to form any suitable shape andstructure corresponding to that of the membrane, porous body, andreactor vessel. Specifically, the catalyzing material can be fabricatedin the shape(s) of disks, tubes, or closed-end tubes. The catalyzingmaterial should be disposed at a distance (D) away from the reductionsurface defined byD=x(T).where x is 0 to 20. In some embodiments of the disclosure, thecatalyzing material and porous body are contiguous, and/or thecatalyzing material and the membrane are contiguous.

Thinner membranes increase the overall flux or diffusion rate for agiven membrane material. To exploit this phenomena, thinner membranesmay be supported by one or more porous bodies.

The support or porous body may be fabricated from an inert materialwhich does not conduct oxygen ions and/or electronic species at processoperating conditions. Alternatively the support can be an ionicallyconducting material, an electron-conducting material or a mixedconducting oxide material of the same or different composition than anactive layer of mixed conducting membrane material. Preferably, theporous support is fabricated from a material having thermal expansionproperties that are compatible with the mixed conducting membranematerial, and the compositions making up the respective layers should beselected from materials that do not adversely chemically react with oneanother under process operating conditions. Preferably, the porous bodyincludes one or more substances selected from the group consisting ofalumina (Al₂O₃), silica (SiO₂), ceria (CeO₂), zirconia (ZrO₂), titania(TiO₂), magnesium oxide (MgO), and mixtures thereof, wherein thesubstance is optionally doped with one or more alkaline earth metals,lanthenum, lanthanide series metals, and mixtures thereof.

In accordance with the preferred embodiments of the disclosure, theporous body should contain a plurality of pores having a mean diameterof at least about five microns. The porous body should have a porosityof about 25% to about 98%, preferably, about 50% to about 95%, and morepreferably about 70% to about 92%.

Unless specified otherwise, the term “oxygen” is used herein to describegenerically any form of oxygen (O, atomic number 8) present in the gasstreams and reactor systems described. The generic term oxygen includesmolecular oxygen (O₂), oxygen ions (for example O⁻ or O²⁻), atomicoxygen (O—), or other forms of oxygen derived from molecular oxygen. Inthe gas streams and systems described. The term “oxygen” as used hereindoes not include oxygen which is chemically bound in oxides of carbon,nitrogen, and sulfur, or other oxygen-containing compounds.

The term “oxygen-containing gas” is used broadly herein to include gasesand mixtures of gases in which at least one of the component gases isoxygen or an oxide. The oxygen or oxide component of the gas is capableof being reduced at the reduction surface of the membrane of thisdisclosure. The term includes oxides of carbon (CO_(x)), nitrogen(NO_(x) and N_(x)O), and sulfur (SO_(x)) among others, and gas mixturesin which an oxide is a component such as, for example, NO_(x) in aninert gas or in another gas not reactive with the membrane. The termalso includes mixtures of oxygen in other gases such as, for example, O₂in air and O in H₂O. In the apparatus of the disclosure, theoxygen-containing gas is passed in contact with the reduction surface ofthe membrane and the oxygen-containing component of the gas is at leastpartially reduced at the reduction surface such as, for example, NO_(x)to N₂. The gas passing out of the reduction zone of the reactor maycontain residual oxygen or oxygen-containing component. “Oxygenselectivity” is intended to convey that the oxygen ions arepreferentially transported across the membrane over other elements, andions thereof.

The term “reactant gas” is used broadly herein to refer to gases ormixtures of gases containing at least one component that is capable ofbeing oxidized at the oxidation surface of the reactor or membranetherein. Reactant gas components include, but are not limited tomethane, natural gas (whose major component is methane), and gaseoushydrocarbons including light hydrocarbons (as this term is defined inthe chemical arts). Those skilled in the art recognize that whilemethane is a major compound of natural gas, other lesser componentsinclude C₃₋₈ hydrocarbons as well as trace amounts of C₂ or higherhydrocarbons. Reactant gases include mixtures of reactant gascomponents, mixtures of such components with inert gases, or mixtures ofsuch components with oxygen-containing species, such as CO, CO₂, or H₂O.The term “oxygen-consuming gas” also may be used herein to describe areactant gas that reacts with oxygen anions generated at the oxidationsurface of the reactor or membrane therein.

The term “oxygen-depleted gas,” dependent upon the context, may refer(1) to a gas or gas mixture from which oxygen has been separated bypassage through a reactor of this disclosure (i.e., the residual of theoxygen-containing gas) or (2) to a gas or gas mixture that is introducedinto the oxidation zone of a reactor used for oxygen separation to carrythe separated oxygen. In the second context, the oxygen-depleted gas maybe an inert gas, air or other non-reactive gas that substantially doesnot contain components that will be oxidized in the oxidation zone ofthe reactor. When used in the second context, the term can be applied tomixtures containing some oxygen, such as air, the oxygen content ofwhich will be increased by passage through the oxidation zone of thereactor.

The terms “oxygen-containing gas,” “reactant gas,” “oxygen-consuminggas,” and “oxygen-depleted gas,” and any other gas mixture discussedherein includes materials which are not gases at temperatures below thetemperature ranges of the pertinent process of the present disclosure,and may include materials which are liquid or solid at room temperature.An example of an oxygen-containing gas which is liquid at roomtemperature is steam.

The term “gas-impermeable” as applied to membrane materials of thisdisclosure means that the membrane is substantially impervious to thepassage of oxygen-containing or reactant gases in the reactor. Minoramounts of transport of gases across the membrane may occur withoutdetriment to the efficiency of the reactor. The meaning of the term “gasimpermeable” is tied to the relative density of a material. For example,a material having a theoretical density greater than 87% is generallyconsidered to be impermeable to gases, assuming that the porosity of thematerial is randomly distributed and there are no cracks in thematerial.

The phrase “different substances” means that that two substanceschemically differ from one another. Thus, for example, materials made ofdifferent elemental compositions are constructed of “differentsubstances,” whereas materials made of the same elemental compositionsbut having different porosity are not constructed of “differentsubstances.” The phrase “different substances” is not meant to excludesituations where the catalyst and the support share common elements(e.g., LaSrFeAlO_(x) on Al₂O₃).

The term “non-reactive” means any initial reaction between twosubstances ceases as the interface between the substances is built upand stabilized. Thus, substances are “non-reactive” with respect to oneanother when a stable interface is established in a short period of timelike, for example, about 24 hours to about 48 hours.

EXAMPLE

The following example is provided to illustrate the disclosure but isnot intended to limit the scope of the disclosure.

A dense, mixed conducting ceramic tube having a closed end and an openend was placed in a reactor system capable of feeding fuelgas on theoutside of the tube and air on the inside of the tube. The tube wasconstructed of LaSrFeCrOx material, and had a length of 12.8 centimeters(cm), an outer diameter of 1.02 cm, and an inner diameter of 0.82 cm.The tube was wrapped in a single layer of 2 cm wide slip of foamed,porous nickel, commercially available as Incofoam™ from Inco SpecialProducts, Toronto, Canada. The strip was wrapped around the tube in ahelical fashion with some overlap between the turns to assure that theentire outer tube surface was covered. The entire reactor system wasplaced in a furnace and heated to about 940° C. The seal between thereactor metal and the ceramic tube was tested to assure that there wasno leakage from the high-pressure fuel side to the low-pressure airside.

Air was fed at atmospheric pressure to the inside of the tube at a rateof about 6 to 8 standard liters per minute (SLPM). Fuel was introducedon the outside of the tube at a pressure of about 185 to about 210pounds per square inch gauge (psig) and at a rate of about 1.7 to about2.5 SLPM. The fuel composition was 10% hydrogen (H₂), 10% carbonmonoxide (CO), 40% methane (CH₄), and 40% carbon dioxide (CO₂). Thesteam to carbon ratio varied from about 0.95 to about 1.26. During therun, the furnace set point was used to control the reaction and wasvaried from about 915° C. to about 942° C. The tube/reactor seal wastested by checking for the pressure of CO₂ in the air stream.

The catalyst temperature was measured at the midpoint of the tube with athermocouple that was located between the foamed, porous nickel catalystand the tube. The catalyst temperature varied during the course of therun from about 750° C. to about 890° C. The fuel-side and air-sideeffluents were analyzed by gas chromatography. The oxygen flux wascalculated to be about 13 standard cubic centimeters (of oxygen) persquare centimeter of membrane surface (sccm/cm²) to about 16 sccm/cm²,based on the air-side oxygen loss and the inner diameter surface area ofthe tube. Methane conversion was greater than 95%. The productdistribution matched the calculated equilibrium values for the giventemperature, pressure, and reactant ratios.

Yield data at various stages in the run are shown in the table below.The values set forth in the table represent raw data and, thus, are notcorrected for mass balances. The flow rates are in sccm units. TABLEYield Data Run Airside Methane Time Temp Pressure Flows In (sccm) FlowsOut (sccm) Conversion (Hours) (° C.) (psig) H₂ CH₄ CO CO₂ H₂O H₂ CH₄ COCO₂ H₂O (%) 0.7 974 188 195 789 155 800 1728 1316 63 970 810 1674 92 40974 208 196 800 202 800 1728 1428 26 1026 823 1674 97 60 986 209 216 881223 880 1965 1512 35 1086 900 1674 96 90 990 183 216 880 223 880 24621565 56 1040 973 2058 94 130 986 185 216 881 223 880 2462 1585 80 982987 2272 91

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom, asmodifications within the scope of the disclosure may be apparent tothose having ordinary skill in the art.

1-23. (canceled)
 24. An apparatus for separating oxygen from anoxygen-containing gas and facilitating chemical reaction with theseparated oxygen, the apparatus comprising: (a) a mixed conductingmembrane having opposed oxidation and reduction surfaces, the membranecomprising a non-porous, gas-impermeable, solid material or mixture ofsolid materials capable of simultaneously conducting oxygen ions andelectrons; and, (b) a porous body comprising a material for catalyzingthe reaction, the body disposed adjacent to the reduction surface,wherein the membrane material and catalyzing material are different fromeach other and are non-reactive with respect to each other at conditionsexperienced during the oxygen separation and the chemical reaction. 25.The apparatus of claim 24 further comprising one or more spacersdisposed between the reduction surface and the porous body, wherein thespacers are non-reactive with the membrane material and the porous body.26. The apparatus of claim 25, wherein the catalyzing material and themembrane are contiguous.
 27. An apparatus for separating oxygen from anoxygen-containing gas and facilitating a chemical reaction with theseparated oxygen, the apparatus comprising: (a) a mixed conductingmembrane having opposed oxidation and reduction surfaces, the membranecomprising a non-porous, gas-impermeable, solid material or mixture ofmaterials capable of simultaneously conducting oxygen ions andelectrons, and, (b) a porous body comprising a material for catalyzingthe reaction, the body disposed adjacent to the reduction surface,wherein the membrane material and catalyzing material are different fromeach other and are not in physical contact with each other.
 28. Theapparatus of claim 27, wherein the catalyzing material and the porousbody, are contiguous.
 29. The apparatus of claim 27, wherein thecatalyzing material and the membrane are contiguous.
 30. The apparatusof claim 27 further comprising one or more spacers disposed between thereduction surface and the porous body, wherein the spacers arenon-reactive with the membrane material and the porous body.
 31. Anapparatus for separating oxygen from an oxygen-containing gas andfacilitating a chemical reaction with the separated oxygen, theapparatus comprising: (a) a mixed conducting membrane having opposedoxidation and reduction surfaces, the membrane comprising a non-porous,gas-impermeable, solid material or mixture of materials capable ofsimultaneously conducting oxygen ions and electrons; (b) a material forcatalyzing the reaction; and, (c) a porous body disposed between thereduction surface and the catalyzing material, wherein the membrane, thecatalyzing material, and the porous body are constructed of differentsubstances.
 32. The apparatus of claim 31, wherein the catalyzingmaterial and porous body are contiguous.
 33. The apparatus of claim 31,wherein the membrane and porous body 143 contiguous.
 34. The apparatusof claim 31, wherein the catalyzing material and the porous body are notin physical contact with each other.
 35. The apparatus of claim 31further comprising one or more spacers disposed between the porous bodyand the catalyzing material, wherein the spacers are non-reactive withthe catalyzing material and the porous body.
 36. An apparatus forseparating oxygen from an oxygen-containing gas and facilitating achemical reaction with the separated oxygen, the apparatus comprising:(a) a multilayered, mixed conducting membrane comprising a non-porouslayer and a porous layer, the membrane comprising a non-porous,gas-impermeable, solid material or mixture of materials capable ofsimultaneously conducting oxygen ions and electrons; and, (b) a materialfor catalyzing the reaction, the catalyzing material disposed withinpores of the porous layer. wherein the catalyzing material isnon-reactive with the membrane material at conditions experienced duringthe oxygen separation and the chemical reaction.
 37. An apparatus forseparating oxygen from an oxygen-containing gas and facilitating achemical reaction with the separated oxygen, the apparatus comprising:(a) a mixed conducting membrane having respective oxidation andreduction surfaces, the membrane comprising a non-porous,gas-impermeable, solid material capable of simultaneously conductingoxygen ions and electrons; (b) a porous body; and (c) a material forcatalyzing the reaction. wherein at least the membrane and thecatalyzing material are non-reactive with each other or are physicallyseparated from each other during oxygen separation and the chemicalreaction.